Recent years have witnessed active research and development efforts in the OLED display and FED fields. Especially, the OLED display is the focus of attention for its light emitting capability at low voltage and power and potential applications in mobile telephones, PDAs (Personal Digital Assistants), and other mobile devices.
OLED displays introduced into the market in initial periods were of simple matrix types. Active matrix types will however be the major player in the future. An OLED active device can be based on amorphous silicon TFTs, mono crystal silicon TFTs, polysilicon TFTs, and CG (Continuous Grain) silicon TFTs. The latter three groups seem to be more promising because they do not require a separate process to fabricate a driver circuit and may be more compact to drive the OLED display (due to high TFT's mobility). Especially preferred among them are low temperature polysilicon TFTs and CG silicon TFTs which can be fabricated on a glass substrate for direct view displays.
A pixel in the active matrix OLED based on a low temperature polysilicon or CG silicon has a basic circuit structure shown in FIG. 23 including two TFTs Qa, Qb, a capacitor Ca, and an OLED ELa. See for example “Active Matrix Addressing of Polymer Light Emitting Diodes Using Low Temperature Polysilicon TFTs (AM-LCD 2000, pp. 249–252).”
The driver TFT Qb is connected in series with the OLED ELa between a power source line Vref and a power source terminal Vcom. The gate and source of the driver TFT Qb sandwich the capacitor Ca. The source of Qb is connected to the power source line Vref. The gate of the selector TFT Qa is connected to a gate line Gi, while its source and drain are connected to couple the source line Sj to the gate of the driver TFT Qb. When the selector TFT Qa is turned on (ON state), the voltage on the source line Sj is applied to the capacitor Ca. The voltage thus controls the on-state resistance of the driver TFT Qb, hence the current through the OLED ELa, which in turn controls the pixel brightness. Thereafter, when the selector TFT Qa is turned off (OFF state), the capacitor Ca maintains its voltage so that the driver TFT Qb continues to be in the conducting state and the pixel brightness remains unchanged.
Applying equal voltages to the capacitors Ca so as to display intermediate tones with this structure possibly ends up with an unwelcome display where brightness varies from pixel to pixel. This is due to variations of the currents through the OLEDs ELa which in turn are caused by variations in threshold value characteristics and mobility of the driver TFTs Qb.
“Active Matrix PolyLED Displays (IDW '00, pp. 235–238)” addresses this problem. The pixel circuit structure is shown in FIG. 24. The circuitry in the figure includes a switching TFT Qc between a driver TFT Qb and an OLED ELa; a selector TFT Qa coupling a source line Sj to the connection of the driver TFT Qb and the switching TFT Qc; and a switching TFT Qd between the switching TFT Qc and a capacitor Ca. The gates of the switching TFTs Qc, Qd are both coupled to the gate line Gi.
In this structure, a current flows from a power source line Vref to the source line Sj when the switching TFT Qc is OFF and the selector TFT Qa and switching TFT Qd are both ON. The current is controlled by a current source in a source driver circuit (not shown) to set the gate voltage of the driver TFT Qb so that the driver TFT Qb conducts the current specified by the source driver circuit regardless of the threshold voltage or mobility of the driver TFT Qb. Then, the selector TFT Qa and the switching TFT Qd are turned off and the switching TFT Qc is turned on, to maintain the voltage across the capacitor Ca at that time. A current as determined by the driver TFT Qb thus flows through the OLED ELa.
FIG. 25 shows another pixel circuit structure. For details, see “Improved Polysilicon TFT Drivers for Light Emitting Polymer Displays (IDW '00, pp. 243–246)” and the Published Japanese Translation of PCT Publication 2002-514320 (Tokuhyo 2002-514320; published on May 14, 2002, a counterpart of PCT application WO98/48403). The circuitry in the figure includes a switching TFT Qg between a driver TFT Qb and a power source line Vref; a switching TFT Qf between the driver TFT Qb and a source line Sj; and a selector TFT Qe between an OLED ELa and a capacitor Ca. The gates of the switching TFTs Qf, Qg and selector TFT Qe are connected to a gate line Gi.
In this structure, a current flows from the source line Sj to the OLED ELa when the switching TFT Qg is turned off and the selector TFT Qe and switching TFT Qf are turned on. The current is controlled by a current driver circuit Pj in a source driver circuit (not shown) to set the gate voltage of the driver TFT Qb so that the driver TFT Qb conducts the current specified by the source driver circuit regardless of the threshold voltage or mobility of the driver TFT Qb. Then, the switching TFT Qf and the selector TFT Qe are turned off, and the switching TFT Qg is turned on, to maintain the voltage across the capacitor Ca at that time. A current as determined by the driver TFT Qb thus flows through the OLED ELa.
FIG. 26 shows another pixel circuit structure. For details, see “13.0-inch AM-OLED Display with Top Emitting Structure and Adaptive Current Mode Programmed Pixel Circuit (TAC) (SID '01, pp. 384–386).” The circuitry in the figure includes a driver TFT Qi between a power source line Vref and a selector TFT Qa; and a switching TFT Qh between the selector TFT Qa and a capacitor Ca. The gate of the selector TFT Qa is connected to a gate line GiA. The gate of the switching TFT Qh is connected to a gate line GiB. The driver TFTs Qb, Qi make up a current mirror circuit having a common gate. The driver TFT Qi is connected to the selector TFT Qa.
A current flows from the power source line Vref to a source line Sj when the selector TFT Qa and switching TFT Qh are turned on. The current is controlled by a current driver circuit Pj in a source driver circuit (not shown) to set the gate voltage of the driver TFT Qi so that the driver TFT Qi conducts a predetermined current regardless of the threshold voltage or mobility of the driver TFT Qi. Then, the switching TFT Qh and selector TFT Qa are turned off, to maintain the voltage across the capacitor Ca at that time. A current as determined by the driver TFT Qb thus flows through the OLED ELa.
See “4.0-in. TFT-OLED Displays and a Novel Digital Driving Method” by Semiconductor Energy Laboratory, SID '00 Digest pp. 924–927, and other publicly available literature for details of CG silicon TFTs; “Continuous Grain Silicon Technology and Its Applications for Active Matrix Display” by Semiconductor Energy Laboratory, AM-LCD 2000 pp. 25–28, and other publicly available literature for details of CG silicon TFT processes; and “Polymer Light-Emitting Diodes for use in Flat panel Display,” AM-LCD '01 pp. 211–214, and other publicly available literature for details of OLED structures.
However, if each source line has a different current source in a TFT-based source driver circuit, the current varies from one source line to another, despite the intended results, due to variations in threshold value characteristics and mobility of the TFTs making up the respective current sources. The TFTs constituting the source driver circuit have properties that may vary from one TFT to another, which leads to variations in their output current and voltage and irregular brightness of the display.
The aforementioned published materials about conventional technology do not disclose the structure of the current driver circuit(s) Pj in the source driver circuit driving the source lines Sj.
A possible approach is to dispose a current driver circuit Pj for each panel or each of RGB. In such structures, the current driver circuits Pj need too high an output current frequency to realize with current TFT's performance.
Another possibility is to fabricate the source driver circuit around a mono crystal IC, not TFTs. This approach fails to make use of the advantage of the low temperature polysilicon TFT and CG silicon TFT that the driver circuit can be fabricated simultaneously as the TFTs.